A lithographic photomask, or reticle, often needs to be re-cleaned during its lifetime. This is particularly true for reticles for lithography with 193 nm wavelength, since they develop crystal growth (so-called haze) during use, which has to be removed by a wet clean typically at intervals of a few months. Another class of reticles that can be expected to need regular re-cleans are the reticles for EUV lithography (which will enter into production in a few years), since these reticles have no protective pellicle (organic membrane on a frame over the active surface) and, therefore, no protection against particles falling onto the patterned surface. Both haze (which is water soluble) and fall-on particles (which are usually much bigger than the structures on the reticle) are relatively easy to remove. However, 193 nm reticles have protective pellicles glued to the front surface, and for re-cleans the pellicle needs to be removed first, which can leave residues of the pellicle adhesive close to the active surface. Reticle cleaning is usually done by dispensing liquid on the patterned surface through a nozzle, mostly with the addition of aggressive chemicals or megasonic power or both. However, the cleaning process itself can leave additional contamination of the surface or damage the patterned surface mechanically, which often leads to costly and time-consuming repair-clean-inspection cycles. Occasionally, the reticle is damaged beyond repair and needs to be replaced. Consequently, re-cleans must be done in mask shops outside the wafer production sites (fabs), which further adds to the cycle time.
Attempts have been made to avoid using adhesive by mechanically attaching the pellicle to the reticle, as disclosed in the copending application entitled, “Mechanical Fixture of Pellicle to Lithographic Photomask,” filed on Dec. 2, 2010, as application Ser. No. 12/958678. Further, since EUV reticles do not have pellicles, the problem of removing glue residuals does not exist for them, but more frequent cleaning is necessary. In both cases, relatively mild cleaning processes would be sufficient to remove haze and/or fall-on particles, while having minimum risk of adding contamination to or damaging the patterned reticle surface.
A need thus exists for methodology and apparatus enabling milder cleaning of a reticle with minimum risk of contamination or damage to the active surface of the reticle.